Household appliances have often a controller which requires a low supply voltage and a power stage which is fed from the mains voltage. The controller switches the power stage on and off and controls the operation thereof. The power stage performs the primary function of the household appliance. The controller responds to, for example, a push button or a signal from a remote control device to switch on and off the power stage. When the household appliance is connected to the mains voltage, the controller only monitors signals that may indicate that the household appliance has to be switched on. The mode wherein the power stage is switched off is called the standby mode. In the standby mode only a small amount of power at a low voltage is consumed to keep the controller and/or an optional remote control sensor awake. When a signal is received to switch on the appliance, the controller enters the operational mode. The power stage is switched on and the controller starts controlling the power stage. Further, the controller may switch on a user-interface to receive additional input from the user and/or to provide feedback to the user. The user-interface is in general powered with the low supply voltage as well. In the operational mode more power than the standby power is consumed by the low supply voltage circuits.
An example of such a household appliance is a coffee maker which may be switched on and off by means of a push button. When the coffee maker is switched on by pushing the on/off push button, the controller switches the coffee maker to the operational mode wherein, depending of the different stages of coffee brewing, the operation of the heating element and, for example, the operation of the water pump is controlled. When the on/off push button is pushed once again, the controller switches off the heating element and/or the water pump and enters the standby mode during which only signals from the push button are monitored.
The low supply voltage is often supplied by a capacitive power supply. A capacitive power supply has a capacitor which provides a capacitive coupling to an AC mains voltage and acts as a charge-pump. A rectifying circuit is used to obtain a DC voltage which is often limited to a low voltage by a zener diode. Depending on the specific configuration of the rectifying circuit only half of the AC mains voltage wave is converted to the low DC supply voltage, or, if full-wave rectification is implemented, the whole AC mains voltage wave is converted. Often a surge resistor is coupled in series with the capacitor and often a bleed resistor is coupled in parallel to the capacitor. The surge resistor protects the power supply against mains voltage peaks and the bleed resistor discharges the capacitor when the power supply is disconnected from the mains voltage.
The capacitive power supply is a relatively efficient power supply because the capacitor does not dissipate power. However, the capacitive power supply can provide a limited amount of power only, because the maximum current that may be delivered is limited by the impedance of the capacitor at the mains voltage frequency. The capacitive power supply is dimensioned for the maximum amount of power that has to be delivered in an operational mode of the household appliance. If, however, less power is consumed by the apparatus, the capacitive power supply, dissipates the surplus energy. Especially in the standby mode too much power is dissipated by the zener diode, because the power which was delivered to the apparatus in the operational mode is dissipated by the zener diode in the standby mode. Further, the surge resistor and the bleed resistor dissipate power in the operational as well as in the standby mode. The power dissipation in, especially, the standby mode is too high and as such the efficiency of the capacitive power supply is too low in the standby mode.
The power dissipation in the surge resistor, the bleed resistor and the zener diode have a linear or quadratic relation with the capacitance of the capacitor. A smaller capacitor results in smaller power losses. Reducing the capacitance of the capacitor is often not possible, because the maximum deliverable power also has a linear relation with the capacitance of the capacitor.